Electromagnetic field shielding device

ABSTRACT

The electromagnetic field shielding apparatus of the present invention includes the electromagnetic field detection coil  21  for detecting an AC leakage electromagnetic field and the parallel resonance circuit  22 - 24  whose circuit constants are predetermined so that the resonance frequency matches the frequency of the electromotive force induced by the electromagnetic field detection coil  21 . The resistor  24  of the parallel resonance circuit  22 - 24  consumes energy so that the strengths of the magnetic and electrical fields passing through the electromagnetic field detection coil  21  are suppressed.

FIELD OF THE INVENTION

The present invention relates to an apparatus which shields anelectromagnetic field emerging from the inside of an electrical device,and in particular to a resonance-type shield and an active shield whichboth use electronic circuitry.

DESCRIPTION OF THE PRIOR ART

Electrical and electronic devices can generate electromagnetic fields.While weak, such fields can make other devices work erroneously or haveadverse effects on the health of users, so that devices need to beshielded.

Shield members made of electric conductors or ferromagnetic materialsare conventionally used to shield the electromagnetic fields (seeJournal of the Electrical Society Vol. 116, April 1996, p203-p217).

FIGS. 21(a) and 21(b) are drawings that show a conventional method forshielding an electromagnetic field with a shield member.

FIG. 21(a) shows a state where an electromagnetic field generated in anelectrical device is emerging from the opening 11 of the chassis 10.

FIG. 21(b) shows a state where the opening 11 is covered with the shieldmember 12 to shield the electromagnetic field.

When the shield member 12 is composed of an electrically conductivematerial, the generated electromagnetic field is reflected back into thedevice chassis by eddy currents which are generated in the shield memberas a reaction to the leakage electromagnetic field. Conversely, when theshield member 12 is composed of a magnetic material that is moremagnetically permeable than air, the magnetic component of theelectromagnetic field is confined within the device by the shieldmember, thereby preventing the electromagnetic field from emerging fromthe opening.

As described below, however, there are problems with this conventionalshield method using the shield member 12.

The first problem is that the opening 11 needs to be covered with asheet or mesh shield member 12 to produce a high shield effect.Therefore, if the opening 11 is an opening which something is insertedinto or removed from, such as a disk slot of a floppy disk drive, or ifthe opening is positioned by a moving mechanism, it is not possible toapply these conventional methods as they are. A complicated shieldmechanism interlocking with the moving mechanism is necessary.

The second problem is that when various kinds of electromagnetic fieldsof different strengths or frequencies are generated in a device or whenthe strength or frequency of an electromagnetic field varies over time,a uniform shield effect cannot be produced with a single shield member12.

The third problem is that when the shield member 12 is composed of amagnetic material, there will be a limit on the strength of the magneticfield which the shield member 12 can shield. For instance, it is verydifficult for a shield member 12 of a magnetic material to shield astrong magnetic field over 1.5 tesla.

With regard to the above problems, the first object of the presentinvention is to provide an electromagnetic field shielding apparatuswhich shields an electromagnetic field that emerges from an opening of achassis without covering the opening, thereby keeping the openingunobstructed.

The second object of the present invention is to provide a flexibleelectromagnetic field shielding apparatus which can easily cope withvarious kinds of electromagnetic fields of different strengths andfrequencies.

The third object of the present invention is to provide anelectromagnetic field shielding apparatus which can produce a uniformshield effect without changing parts or materials of the apparatus evenif the strength or frequency of the electromagnetic field varies overtime.

The fourth object of the present invention is to provide anelectromagnetic field shielding apparatus which can shield a strongmagnetic field over 1.5 tesla.

SUMMARY OF THE INVENTION

The resonance-type electromagnetic field shielding apparatus of thepresent invention includes an electromagnetic field detection meanswhich includes a coil for detecting an electromagnetic field, and aresonance means which consumes electricity, where the circuit constantsof respective parts of the resonance means are predetermined so that theresonance means is tuned to the frequency of the electromotive forceinduced by the electromagnetic field detection means.

With this apparatus, the electromagnetic field detection means detectsan electromagnetic field, an electromotive force is induced in the coil,the electromotive force is supplied to the resonance means, and theenergy of the electromotive force is consumed therein. As a result, thestrength of the electromagnetic field is suppressed, thereby achieving ashield effect.

By providing the electromagnetic field detection means including a coilaround the edge of an opening of an electrical device from which anunnecessary electromagnetic field is emerging, the strength of theelectromagnetic field passing through the electromagnetic fielddetection means is suppressed. Therefore, the electromagnetic fieldemerging from the opening is shielded without covering the opening tokeep it open, thereby achieving the first object.

Here, the resonance means may be a parallel resonance circuit includingL, C, and R electronic parts or may be an electric conductor which canbe used as a distributed constant circuit.

With this structure, by changing these circuit parts (the electronicparts or the electric conductor), a resonance means causing a differentenergy consumption or a resonance means having a different resonancefrequency can be obtained without difficulty, thereby achieving thesecond and fourth objects.

The L, C, and R electronic parts may be circuit parts with variablecircuit constants, and the electromagnetic field shielding apparatus maybe provided with a means for identifying a center frequency of theelectromotive force and a means for determining and controlling thecircuit constants to produce resonance at the identified centerfrequency.

With this structure, the apparatus changes the resonance frequency tofollow variation in the frequency or strength of an electromagneticfield, or changes the amount of the energy consumption, therebyachieving the third object.

The electromagnetic field shielding apparatus of the present inventionmay be an active electromagnetic field shielding apparatus whichincludes an electromagnetic field detection means for detecting anelectromagnetic field, an electromagnetic field generation means forgenerating a counteractive electromagnetic field, and a control meansfor controlling the electromagnetic field generation means so that theelectromagnetic field generation means generates a counteractiveelectromagnetic field for canceling out the electromagnetic fielddetected by the electromagnetic field detection means.

With this apparatus, the electromagnetic field generation meansgenerates an electromagnetic field to counteract the leakageelectromagnetic field detected by the electromagnetic field detectionmeans. Therefore, by making the electromagnetic field detection meansand the electromagnetic field generation means target the same space,the electromagnetic field in the space is canceled out.

Here, the electromagnetic field detection means and the electromagneticfield generation means may be hollow coils whose hollow parts areidentical or have identical center axes, and may be provided around theedge of the opening of an electrical device from which anelectromagnetic field is emerging.

In this manner, the electromagnetic field emerging from the inside of anelectrical device through its opening is canceled out, thereby achievingthe first object.

Also, by selecting the constants of the circuit parts appropriately orby replacing the circuit parts with parts having variable circuitconstants, the second and third objects are achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the appearance of the electromagnetic field shieldingapparatus 20 of Embodiment 1;

FIG. 2 is a circuit diagram of the electromagnetic field shieldingapparatus 20;

FIG. 3 shows the overall appearance of the chassis 10 of an electricaldevice and the electromagnetic field shielding apparatus 20, where theelectromagnetic field shielding apparatus 20 is provided on the outsideof the chassis 10 of the electrical device around the edge of theopening 11;

FIG. 4(a) shows the overall appearance of the chassis 10 of anelectrical device and the electromagnetic field shielding apparatus 20,where the electromagnetic field shielding apparatus 20 is provided onthe inside of the chassis 10 of the electrical device;

FIG. 4(b) is a sectional view taken along line A—A of FIG. 4(a);

FIGS. 5(a) to 5(c) show a simulation model used to verify the shieldeffect of the electromagnetic field shielding apparatus 20, where FIG.5(a) is a perspective view, FIG. 5(b) is a top end view, and FIG. 5(c)is a sectional view taken along line B—B of FIG. 5(b);

FIG. 6 shows the distribution of the leakage flux lines in the sectionalview taken along line B—B of FIG. 5(b), when the model shown in FIGS.5(a) to 5(c) is not provided with the electromagnetic field shieldingapparatus 20;

FIG. 7 shows the distribution of the leakage flux lines in the sectionalview taken along line B—B of FIG. 5(b), when the model shown in FIGS.5(a) to 5(c) is provided with the electromagnetic field shieldingapparatus 20;

FIG. 8 is a graph of the shield effect on the leakage electromagneticfield when the inductance of the coil 23 of the electromagnetic fieldshielding apparatus 20 is changed;

FIG. 9 shows the appearance of the electromagnetic field shieldingapparatus 40 of Modification 1 of Embodiment 1;

FIG. 10 shows the distributed constant circuit used to shows the shieldprinciple of the electromagnetic field shielding apparatus 40;

FIG. 11 is a standardized circuit diagram of the resonance-typeelectromagnetic field shielding apparatus and the electromagnetic fieldshielding apparatus using the distributed constant circuit of thepresent invention;

FIG. 12 is a block diagram of the electromagnetic field shieldingapparatus 30 of Modification 2 of Embodiment 1;

FIG. 13 is a block diagram showing the detailed structure of thevariable reactor 34 of the electromagnetic field shielding apparatus 30;

FIG. 14 is a block diagram showing the detailed structure of the circuitconstant control unit 33 of the electromagnetic field shieldingapparatus 30;

FIG. 15 shows the appearance of the electromagnetic field shieldingapparatus 50 of Embodiment 2;

FIG. 16 is a block diagram of the electromagnetic field shieldingapparatus 50;

FIG. 17(a) shows the waveform of the induced electromotive force Vsgenerated in the electromagnetic field detection unit 51 of theelectromagnetic field shielding apparatus 50;

FIG. 17(b) shows the waveform of the signal of the center frequencycomponent of the induced electromotive force Vs (the dashed line) andthe waveform of the filter signal V0 output from the band-pass filter 53a (the solid line);

FIG. 17(c) shows the waveform of the delayed signal V1 output from thedelay unit 53 b (and the voltage waveform V2 output from the poweramplification unit 53 c);

FIG. 18 shows an example of the electromagnetic field shieldingapparatus 20 provided on the inside of the front of a CRT device;

FIG. 19(a) shows an example of the electromagnetic field shieldingapparatus 20 provided on the inside of the front of a mobile phone;

FIG. 19(b) is a sectional view taken along line C—C of FIG. 19(a);

FIG. 20(a) shows an example of the electromagnetic field shieldingapparatus 20 provided around the disk slot of the floppy disk drive of apersonal computer;

FIG. 20(b) is a sectional view taken along line D—D of FIG. 20(a);

FIG. 21(a) shows the state where an electromagnetic field generated inan electrical device is emerging from the opening 11 of the chassis 10;and

FIG. 21(b) is a drawing used to describe the conventional techniquewhere the opening 11 is covered with the shield member 12 to block theelectromagnetic field.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following is a detailed description of embodiments of the presentinvention, with reference to the drawings.

<Embodiment 1>

Embodiment 1 is the resonance-type electromagnetic field shieldingapparatus which uses electronic circuitry.

FIG. 1 shows the appearance of the electromagnetic field shieldingapparatus 20 of Embodiment 1.

FIG. 2 is a circuit diagram of the electromagnetic field shieldingapparatus 20.

FIG. 3 shows the overall appearance of the chassis 10 of an electricaldevice and the electromagnetic field shielding apparatus 20, where theelectromagnetic field shielding apparatus 20 is provided on the outsideof the chassis 10 to shield an electromagnetic field emerging from theopening 11 of the chassis 10 of the electrical device.

FIG. 4(a) shows the overall appearance of the chassis 10 of anelectrical device and the electromagnetic field shielding apparatus 20,where the electromagnetic field shielding apparatus 20 is provided onthe inside of the chassis 10 of the electrical device.

FIG. 4(b) is a sectional view taken along line B—B of FIG. 4(a).

The electromagnetic field shielding apparatus 20 includes theelectromagnetic field detection coil 21, the capacitor 22, the coil 23,the resistor 24, and the circuit board 25, where the capacitor 22, thecoil 23, and the resistor 24 are mounted on the circuit board 25.

The electromagnetic field detection coil 21 is a hollow coil fordetecting an electromagnetic field and is an enamel coating copper wirewhich is wound in an L shape to surround the opening 11.

The induced electromotive force Vs, which is expressed by Formula 1below and is proportionate to the variation in the magnetic flux Φpassing through the electromagnetic detection coil 21, is generated inthe electromagnetic field detection coil 21. $\begin{matrix}{{Vs} = {{- N}\frac{\varphi}{t}}} & \left( {{Formula}\quad 1} \right)\end{matrix}$

where N is the number of turns of the electromagnetic field detectioncoil 21.

As is apparent from Formula 1, the electromagnetic field shieldingapparatus 20 only detects variation in a leakage “flux” (an AC leakageflux), it may be said that the electromagnetic field shielding apparatus20 detects an “electromagnetic field” because an electric field is alsogenerated at the place where magnetic flux varies.

The capacitor 22, the coil 23, and the resistor 24 which are mountedonto the circuit board 25 act as the load when the electromotive forceVs induced in the electromagnetic field detection coil 21 is the signalsource to the load. The elements construct a parallel resonance circuitwhose resonance frequency f0 is expressed by Formula 2 below.$\begin{matrix}{{f0} = {\frac{1}{2\pi}\sqrt{\frac{1}{L1C} - \frac{R^{2}}{{L1}^{2}}}}} & \left( {{Formula}\quad 2} \right)\end{matrix}$

where C is the capacitance (F) of the capacitor 22, L1 the inductance(H) of the coil 23, and R the resistance value (Ω) of the resistor 24.The specific values of these C, L1, and R are predetermined so thatcenter frequency (the frequency of a signal component having the largestquantity in the frequency spectrum) of the leakage magnetic flux whichis the target of the electromagnetic field shielding apparatus 20matches the resonance frequency f0 expressed by Formula 2 above. Forinstance, the combined use of a 0.01 μF capacitor 22, a 10 μH coil 23,and a 10Ω resistor 24 achieves an electromagnetic field shieldingapparatus which shields a 478 KHz electromagnetic field.

The following is a description of the technical basis of the shieldeffect produced by the electromagnetic field shielding apparatus 20.

When an AC electromagnetic field passes through the electromagneticfield detection coil 21, the AC-induced electromotive force Vs shown inFormula 1 is generated by variation in a magnetic flux in theelectromagnetic field. This AC signal Vs is supplied to the pre-tunedparallel resonance circuit including the elements 22-24 (hereinafterthis circuit is called “the parallel resonance circuit 22-24” or “theresonance circuit 22-24”). In the resonance circuit 22-24, resonance iscaused, generating electricity which is accumulated in the capacitor 22.

The resistor 24 with positive resistance is inserted into the resonancecircuit 22-24 to consume electricity. The consumption of electricity bythe resistor 24 means the consumption of the electromotive force inducedin the electromagnetic field detection coil 21, that is, the release ofenergy of the leakage electromagnetic field. Therefore, the consumptionof the electricity by the resistor 24 achieves the result that theelectromagnetic field is shielded.

That is to say, because the electromotive force Vs induced by theelectromagnetic detection coil 21 is attenuated by the resistor 24 inthe parallel resonance circuit, the variation in the magnetic flux Φpassing through the electromagnetic detection coil 21 is suppressed andvanishes. It is apparent from Formula 3 below, which is one of Maxwell'sequations, that, when there is no variation in a magnetic flux Φ (namelyvariation in the magnetic field H or variation in flux density B) in anAC electromagnetic field, the electric field E is also zero in theelectromagnetic field detection coil 21. $\begin{matrix}{{rotE} = {{- \frac{B}{t}}\quad \left( {{{where}\quad B} = {\mu \quad H}} \right)}} & \left( {{Formula}\quad 3} \right)\end{matrix}$

Therefore, the magnetic field and the electric field in theelectromagnetic field detection coil 21, that is, the electromagneticfield that attempts to pass through the electromagnetic field detectioncoil 21 can be shielded.

The following is a description of results of a simulation of the shieldeffect of the electromagnetic field shielding apparatus 20 having theabove structure.

FIGS. 5(a) to 5(c) show the simulation model used to verify the shieldeffect of the electromagnetic field shielding apparatus 20, where FIG.5(a) is a perspective view, FIG. 5(b) is a top end view, and FIG. 5(c)is a sectional view taken along line B—B of FIG. 5(b).

This model includes the coil 13 which generates a radiationelectromagnetic field of 100 KHz, the lower chassis 10 a which containsthe coil 13 and is open upward, the upper chassis 10 b which is placedover the lower chassis 10 a with the opening gap 14 therebetween, andthe electromagnetic field shielding apparatus 20 which is placed toshield the electromagnetic field that emerges from the front of theopening gap 14.

The lower chassis 10 a and the upper chassis 10 b are originally arectangular chassis except for its top and the top, respectively, andare made of magnetic substances.

FIG. 6 shows the distribution of the leakage flux lines in the sectionalview taken along line B—B of FIG. 5(b), when the simulation model is notprovided with the electromagnetic field shielding apparatus 20.

FIG. 7 shows the distribution of the leakage flux lines in the sectionalview taken along line B—B of FIG. 5(b), when the simulation model isprovided with the electromagnetic field shielding apparatus 20.

It is apparent from the FIGS. 6 and 7 that the leakage flux linesleaking from the chassis 10 a and 10 b are abated by the electromagneticfield shielding apparatus 20.

FIG. 8 is a graph showing the shield effect on the electromagneticfield, when the capacitance of the capacitor 22 and the resistance valueof the resistor 24 are fixed at constant values and the inductance ofthe coil 23 is changed in the electromagnetic field shielding apparatus20. The vertical axis shows the shield effect on the leakageelectromagnetic field, namely the strength of the magnetic field whenthe electromagnetic field shielding apparatus 20 is provided, on theassumption that the strength of the leakage magnetic field without theelectromagnetic field shielding apparatus 20 is one.

It is apparent from FIG. 8 that the profoundest effect is produced bytuning the resonance frequency of the parallel resonance circuit 22-24in the electromagnetic field shielding apparatus 20 to the frequency ofthe AC electromagnetic field generated in the chassis 10 a and 10 b.

As described above, the resonance-type electromagnetic field shieldingapparatus 20 of Embodiment 1 of the present invention shields theelectromagnetic field passing through the electromagnetic fielddetection coil 21. Furthermore, because this electromagnetic fielddetection coil 21 can be formed in any shape, it is possible to shieldthe electromagnetic field emerging from an opening of a device chassiswithout covering the opening, thereby keeping the opening unobstructed.

Also, by appropriately selecting the electronic parts 22-24 of theparallel resonance circuit, an electromagnetic field shielding apparatus20 which corresponds to various kinds of AC electromagnetic fields ofdifferent frequencies or strengths can be constructed withoutdifficulty. For instance, by appropriately selecting the circuitconstant, a strong magnetic field over 1.5 tesla can be shielded.

<Modification 1>

The following is a description of the electromagnetic field shieldingapparatus 40 using a distributed constant circuit which is Modification1 of the electromagnetic field shielding apparatus 20 of Embodiment 1.

FIG. 9 shows the appearance of the electromagnetic field shieldingapparatus 40 of Modification 1.

The electromagnetic field shielding apparatus 40 includes theelectromagnetic field detection coil 21 and the plate-shaped electricconductor 41 and is a shielding apparatus for an AC electromagneticfield of a higher frequency than in Embodiment 1, which is a highfrequency at which the apparatus itself can be used as a distributedconstant circuit.

The electric conductor 41 is an aluminum plate, for instance. The endparts 41 a and 41 b, which are connected to the electromagnetic fielddetection coil 21, are placed to face each other, and the main part 41 cexcept for the end parts 41 a and 41 b is bent in rectangular form.

The electric conductor 41 has a resistance value which is determined byits metal material, the end parts 41 a and 41 b have capacitance becausethey face each other, and the main part 41 c corresponds to a one-turncoil. Therefore, it may be said that the electric conductor 41constructs a cavity resonator corresponding to the resonance circuit22-24 of Embodiment 1. As a result, the electromagnetic field shieldingapparatus 40 of Modification 1 produces the same shield effect asEmbodiment 1 on a high frequency leakage electromagnetic field.

The following is a description of the shield principle of the shieldingapparatus using a distributed constant circuit, such as theelectromagnetic field shielding apparatus 40, and conditions forproducing a shield effect with this shielding apparatus.

As shown in FIG. 10, the electromagnetic field shielding apparatus 40can be regarded as a distributed constant circuit including the highfrequency power source 42 corresponding to the electromagnetic fielddetection coil 21, the lines 43 transmitting electric power of the highfrequency power source 42, and the load 44 connected to the ends.

Assuming that the direction from the power source 42 to the load 44 isthe positive direction on x-axis , voltage V(x) and current I(x) atpoint x of a distributed constant circuit are expressed by Formula 4 andFormula 5 below.

V(X)=V1e^(−γX)+V2e^(+γX)  (Formula 4)

$\begin{matrix}{{I(X)} = {\frac{\gamma}{Z}\left( {{{V1}\quad ^{{- \gamma}\quad X}} - {{V2}\quad ^{{+ \gamma}\quad X}}} \right)}} & \left( {{Formula}\quad 5} \right)\end{matrix}$

where Z is impedance per unit length of the lines and γ is a propagationconstant (α+jβ, where α is an attenuation constant and β is a phaseconstant)

Here, on the assumption that the line 43 is a zero-loss line (α=0),Formula 4 and Formula 5 become Formula 6 and Formula 7, respectively.

V(X)=V1e^(−jβX+V2e) ^(jβX)  (Formula 6)

$\begin{matrix}{{I(X)} = {\frac{1}{Z0}\left( {{V1}^{{- {j\beta}}\quad X} - {V2}^{{j\beta}\quad X}} \right)}} & \left( {{Formula}\quad 7} \right) \\{{{where}\quad {characterisic}\quad {impedance}\quad {Z0}} = {\frac{Z}{\gamma} = \sqrt{\frac{L}{C}}}} & \quad \\{\beta = {\omega \sqrt{LC}}} & \quad\end{matrix}$

β=ω{square root over (LC)}

When the length of the lines 43 is A and impedance of the load 44connected to the ends of the lines 43 is equal to characteristicimpedance Z0 of the lines 43, Formula 8 and Formula 9 below should holdat the ends. $\begin{matrix}{\frac{V(A)}{I(A)} = {Z0}} & \left( {{Formula}\quad 8} \right) \\{\frac{V(A)}{I(A)} = {{Z0}\quad \frac{{{V1}\quad ^{{- {j\beta}}\quad A}} + {{V2}\quad ^{{j\beta}\quad A}}}{{{V1}\quad ^{{- {j\beta}}\quad A}} - {{V2}\quad ^{{j\beta}\quad A}}}}} & \left( {{Formula}\quad 9} \right)\end{matrix}$

From Formula 8 and Formula 9, V2 equals to 0 so that there is noreflected wave V2exp (jbx).

That is to say, by matching the impedance of the lines 43 to that of theload 44, electric power supplied from the power source 42 is consumed bythe load 44 without being reflected.

As described above, by matching the impedance of the AC electromagneticfield targeted by the electromagnetic field shielding apparatus 40 tothose of the lines and load, a high-frequency electromagnetic can beshielded. This principle is fundamentally the same as that of theelectromagnetic field shielding apparatus 20 of Embodiment 1 whichshields an electromagnetic field of a comparatively low frequency byconnecting the AC electromagnetic field targeted by the shieldingapparatus 20 to the resonance circuit 22-24.

The following is a description of the general shield principle andconditions for producing a shield effect in Embodiment 1 andModification 1.

FIG. 11 is a standardized circuit diagram of the resonance-typeelectromagnetic field shielding apparatus and the electromagnetic fieldshielding apparatus using a distributed constant circuit of the presentinvention.

These electromagnetic field shielding apparatuses 20 and 40 may beconsidered to have the structure including the induced electromotiveforce 42 induced by an electromagnetic field, the load 44, and thefour-terminal network 45 by which the induced electromotive force 42 andthe load 44 are connected together.

In Formula 10 below, each parameter (S11, S12, S21, and S22) isdetermined so that S11 (voltage reflection coefficient) is as close aspossible to zero, with V2=−RI2 holding. $\begin{matrix}{\left\lfloor \begin{matrix}{V1} \\{I1}\end{matrix} \right\rfloor = {\left\lfloor \begin{matrix}{S11} & {S12} \\{S21} & {S22}\end{matrix} \right\rfloor \left\lfloor \begin{matrix}{V2} \\{I2}\end{matrix} \right\rfloor}} & \left( {{Formula}\quad 10} \right)\end{matrix}$

The electromagnetic field shielding apparatus of the present inventionis achieved by placing the four-terminal network 45 with thecharacteristic expressed by the parameters determined as described abovebetween the load 44 and the electromagnetic field detection coil 21.

As described above, the parallel resonance electromagnetic fieldshielding apparatus 20 using integrated constant elements and theimpedance matching electromagnetic field shielding apparatus 40 using adistributed constant circuit have a common principle that anelectromagnetic field is shielded by transferring energy of the powersource 42 in the four-terminal network 45 in one direction and consumingthe energy using the load 44.

<Modification 2>

The following is a description of the electromagnetic field shieldingapparatus 30 which shields a varying electromagnetic field by followingthe variation in this leakage electromagnetic field dynamically, asModification 2 of the electromagnetic field shielding apparatus 20 ofEmbodiment 1.

FIG. 12 is a block diagram of the electromagnetic field shieldingapparatus 30 of Modification 2.

The electromagnetic field shielding apparatus 30 includes theelectromagnetic field detection coil 21, the waveform analyzer 32, thecircuit constant control unit 33, the variable reactor 34, the variablecapacitor 35, and the variable resistor 36. The apparatus 30 andEmbodiment 1 are equivalent in being resonance shielding apparatuses,but differ in that the apparatus 30 has the function for being tuned tothe frequency of an electromagnetic field detected by theelectromagnetic field detection coil 21.

The waveform analyzer 32 includes a DSP (Digital Signal Processor) forperforming FFT (Fast Fourier Transformation). The waveform analyzer 32identifies the center frequency f0 of the electromotive force Vs inducedby the electromagnetic field detection coil 21 at a constant timeinterval and informs the circuit constant control unit 33 of the centerfrequency f0.

As shown in FIG. 13, the variable reactor 34 includes MOS transistors 37a-37 d which are a plurality of switches, the coils 38 a-38 d, and thedecoder circuit 39 connected to respective gates of the MOS transistors37 a-37 d. The variable reactor 34 changes an inductance in stages byturning ON only one out of the MOS transistors 37 a-37 d according tothe instruction from the circuit constant control unit 33.

The variable capacitor 35 is, for instance, a variable capacity diodeand its capacity is changed in a specific range by the control voltagefrom the circuit constant control unit 33.

The variable resistor 36 is, for instance, a MOS transistor and itsresistance value is changed in a specific range by the control voltagefrom the circuit constant control unit 33.

The circuit constant control unit 33 controls the respective circuitconstants of the electronic parts 34-36 according to Formula 2 above, sothat the parallel resonance circuit including the elements 34-36 istuned to the center frequency f0 indicated by the waveform analyzer 32.

FIG. 14 is a block diagram showing a detailed structure of the circuitconstant control unit 33.

The circuit constant control unit 33 includes the circuit constantstorage table 33 b for storing inductances to be selected for eachcenter frequency f0 informed from the waveform analyzer 32 and anadjustable circuit constant range of each electronic circuit, circuitconstant calculating unit 33 a for calculating the respective optimalcircuit constants of the electronic parts 34-36 according to the circuitconstant storage table 33 b, to the center frequency f0 informed fromthe waveform analyzer 32, and to Formula 2 above, and the D/A converters33 c and 33 d for converting the calculated digital data into a controlvoltage.

With the electromagnetic field shielding apparatus 30 having the abovestructure of Modification 2, even if the frequency of the ACelectromagnetic field passing through the electromagnetic fielddetection coil 21 varies, the circuit constants of circuit parts 34-36are adjusted and are tuned following the varying frequency, so that aconstant shield effect is always produced. This means that theelectromagnetic field shielding apparatus 30 produces a constant shieldeffect without requiring changes to its construction or constituentmaterials in response to changes in the frequency of the electromagneticfield to be shielded.

<Embodiment 2>

Embodiment 2 relates to an active shield including an electromagneticfield generation means for canceling out an electromagnetic field.

FIG. 15 shows the appearance of the electromagnetic field shieldingapparatus 50 of Embodiment 2.

The electromagnetic field shielding apparatus 50 includes theelectromagnetic field detection unit 51, the electromagnetic fieldgeneration unit 52, and the electromagnetic field control unit 53.

The electromagnetic field detection unit 51 is the same as theelectromagnetic field detection coil 21 of Embodiment 1.

The electromagnetic field generation unit 52 is a hollow coil, whoseconductor is wound in the same form and direction as the electromagneticfield detection unit 51. The electromagnetic field generation unit 52 isfirmly attached to or is placed over the electromagnetic field detectionunit 51 so that the spaces surrounded by both units are coincident or atleast coaxial, and generates a counteractive electromagnetic field forcanceling out a leakage electromagnetic field passing through theelectromagnetic field detection unit 51.

The magnetic field control unit 53 is a control circuit for generatingthe reaction magnetic field by generating an electromotive force with aphase opposite to an induced electromotive force generated in theelectromagnetic field detection unit 51 and by supplying theelectromotive force to the electromagnetic field generation unit 52.

The apparatus 50 is mounted on an electrical apparatus in the mannershown in FIG. 3 or in FIGS. 4(a) and 4(b).

FIG. 16 is a block diagram of the electromagnetic field shieldingapparatus 50.

The electromagnetic field control unit 53 further includes the band-passfilter 53 a, the delay unit 53 b, and the power amplification unit 53 c.

FIGS. 17(a) to 17(c) are waveform diagrams for showing relations amongphases of signals output from respective components. FIG. 17(a) showsthe waveform of the induced electromotive force Vs generated in theelectromagnetic field detection unit 51; FIG. 17(b) shows the waveformof the signal of the center frequency component of the inducedelectromotive force Vs (the dashed line) and the waveform of the filtersignal V0 output from the band-pass filter 53 a (the solid line); andFIG. 17(c) shows the waveform of the delayed signal V1 output from thedelay unit 53 b (this waveform has the same phase as the voltagewaveform V2 output from the power amplification unit 53 c).

The band-pass filter 53 a is an active filter including an operationalamplifier or the like for removing unnecessary frequency components outof the induced electromotive force Vs generated in the electromagneticfield detection unit 51. After the induced electromotive force Vsgenerated in the electromagnetic field detection unit 51 is amplified,the band-pass filter 53 a passes and transmits to the delay unit 53 bonly the frequency component V0 whose band is the predetermined shieldtarget.

The delay unit 53 b includes a delay line for delaying the phase of theinput signal V0 so that the electromagnetic field generated by theelectromagnetic field generation unit 52 cancels out the leakageelectromagnetic field passing through the electromagnetic fielddetection unit 51. For instance, if the band-pass filter 53 a outputsthe center frequency component V0 delayed by td1, the delay unit 53 bdelays the output center frequency component V0 by td2 to cause a phasedelay of 180° in total.

The power amplification unit 53 c is an AC amplifier for amplifying theelectric power of the signal output from the delay unit 53 b andsupplies an alternating current for generating a counteractiveelectromagnetic field in the electromagnetic field generation unit 52.

The following is a description of the operation of the electromagneticfield shielding apparatus 50 having the above structure, according toformulas.

The induced electromotive force Vs, which is proportional to variationin the magnetic flux that passes through the coil and is expressed byFormula 11 below, is generated in the electromagnetic field detectionunit 51. $\begin{matrix}{{{Vs}(t)} = {{- N}\frac{\varphi}{t}}} & \left( {{Formula}\quad 11} \right)\end{matrix}$

The band-pass filter 53 a passes only the frequency component V0 with aspecific band (f0) out of the induced electromotive force Vs, causes adelay of td1, and outputs the signal V0 expressed by Formula 12 below.

V0(t)=[Vs (t−td1)]f=f0  (Formula 12)

By delaying the input signal V0 by td2, the delay unit 53 b generatesthe signal V1 expressed by Formula 13, which is the center frequencycomponent of the induced electromotive force Vs delayed by the phase of180°. $\begin{matrix}\begin{matrix}{{{V1}(t)} = {{V0}\left( {t - {td2}} \right)}} \\{= {- {{Vs}(T)}}}\end{matrix} & \left( {{Formula}\quad 13} \right)\end{matrix}$

The power amplification unit 53 c generates the signal V2 expressed byFormula 14 which is the delay signal V1 amplified by k times so that anelectromagnetic field having the same strength as the electromagneticfield is generated in the electromagnetic field generation unit 52.

V2(t)=−kVs(t)  (Formula 14)

As described above, with the electromagnetic field shielding apparatus50 of Embodiment 2, the electromagnetic field having a specificfrequency detected by the electromagnetic field detection unit 51 iscanceled out by the electromagnetic field generated by theelectromagnetic field generation unit 52 located over theelectromagnetic field detection unit 51. As a result, theelectromagnetic field within the electromagnetic field detection unit 51vanishes, with the electromagnetic field being shielded.

With the electromagnetic field shielding apparatus of Embodiment 2, asin Embodiment 1, coils of the electromagnetic field detection unit 51and the electromagnetic field generation unit 52 can be formed in anyshape so that the electromagnetic field emerging from an opening of adevice chassis can be shielded without covering the opening, therebykeeping the opening unobstructed.

Furthermore, by appropriately selecting the band which the band-passfilter 53 a allows to pass, the delay time of the delay unit 53 b, andthe amplification factor of the power amplification unit 53 c, an activeshield corresponding to various kinds of AC electromagnetic fieldshaving different frequencies or strengths can be constructed withoutdifficulty. For instance, a strong magnetic field over 1.5 tesla can beshielded.

It should be noted here that although the electromagnetic fielddetection unit 51 is the coil for detecting an AC electromagnetic fieldin Embodiment 2, the present invention is not restricted to thisstructure and, for instance, the electromagnetic field detection unit 51may be a vertical antenna or a Hall element which detects AC as well asDC magnetic fields. Various kinds of electromagnetic fields can beshielded when the electromagnetic field control unit 53 performs acontrol corresponding to the characteristic of the electromagnetic fielddetection unit 51.

Also, while the electromagnetic field detection unit 51 and theelectromagnetic field generation unit 52 have individual coils, one coilmay double as these coils. In this case, by providing in anelectromagnetic field a coil doubling as the detection and generationcoils of an electromagnetic field and by controlling the power supply tothe coil, the electromotive force Vs induced in the coil becomes zero.

Still, while the electromagnetic field shielding apparatus 50 ofEmbodiment 2 targets an electromagnetic field with a specific frequency,the electromagnetic field shielding apparatus 50 may be a dynamic activeshield following variation in an electromagnetic field by furtherincluding the waveform analyzer 32 and the circuit constant control unit33, like the electromagnetic field shielding apparatus 30 ofModification 2 of Embodiment 1.

Furthermore, while one electromagnetic field shielding apparatus forshielding an electromagnetic field is used in Embodiments 1 and 2, aplurality of electromagnetic field shielding apparatuses correspondingto the distribution of an electromagnetic field may be installed. Oneelectromagnetic field shielding apparatus may includes a plurality ofelectromagnetic field detection coils or a plurality of electromagneticfield generation coils.

<Application Example in Electrical Device>

The following is a description of examples of the electromagnetic fieldshielding apparatuses 20, 30, 40, and 50 of Embodiments 1 and 2 andModifications 1 and 2 applied to an electronic or electrical device.

FIG. 18 shows an example of the electromagnetic field shieldingapparatus 20 provided on the inside of the front of a CRT device.

With this structure, an unnecessary electromagnetic field, which isgenerated by the deflecting yoke or high-frequency flyback transformerin a CRT device and emerges from the display screen, is shielded so thatuser health problems caused by the electromagnetic field can beprevented.

FIGS. 19(a) and 19(b) show an example of the electromagnetic fieldshielding apparatus 20 provided on the inside of the front of a mobilephone.

With this structure, an unnecessary electromagnetic field radiatedforward from a PLL synthesizer or transmission amplifier in the cellularphone is suppressed, reducing the influence of such fields on the healthof users.

FIGS. 20(a) and 20(b) show an example of the electromagnetic fieldshielding apparatus 20 provided around the disk slot of the floppy diskdrive of a personal computer.

With this structure, an electromagnetic field emerging from a disk slotof a personal computer can be shielded easily without covering the diskslot.

As described above, with the resonance-type electromagnetic fieldshielding apparatus and the active shield of the present invention, anelectromagnetic field emerging from an opening of a device chassis isshielded without covering the opening to keep it open. A shieldingapparatus, which can easily cope with various kinds of electromagneticfields having different strengths and frequencies without changingcomponents or raw materials of the apparatus, is produced, and an evenshield effect is produced even if the strength or frequency of a leakageelectromagnetic field varies over time.

As a result, adverse influence on human bodies and erroneous operationof devices due to the electromagnetic radiation from an electricaldevice are prevented and the bearing capacity against externalelectromagnetic interference, namely “immunity,” is improved so that theshielding apparatus of the present invention has an enormous practicalvalue as an EMC (Electromagnetic Compatibility) appliance.

Industrial Applicability

As described above, the electromagnetic field shielding apparatus of thepresent invention is used as a device for shielding an electromagneticfield emerging from an electrical or electronic device, such as acomputer display device or a mobile phone, and as an apparatus forshielding an electromagnetic field emerging from an apparatus generatinga strong magnetic field, such as MRI (Magnetic Resonance Imaging). Inparticular, the electromagnetic field shielding apparatus is suitable asan electronic device for easily shielding an electromaganetic fieldemerging through a disk slot of a floppy disk drive or through anopen/close door without covering the opening of the disk slot or thedoor.

What is claimed is:
 1. An apparatus which shields an electromagneticfield, the electromagnetic field shielding apparatus being characterizedby comprising: an electromagnetic field detection means for detecting anelectromagnetic field and converting the electromagnetic field into anelectric signal; and a parallel resonance means for causing a parallelresonance which consumes power of the electric signal.
 2. Theelectromagnetic field shielding apparatus defined in claim 1, whereinthe electromagnetic field detection means is a coil.
 3. Theelectromagnetic field shielding apparatus defined in claim 2, whereinthe parallel resonance means includes an inductance element, acapacitance element, and a resistance element.
 4. The electromagneticfield shielding apparatus defined in claim 3, wherein circuit constantsof the inductance element, the capacitance element, and the resistanceelement are variable, and the electromagnetic field shielding apparatusfurther comprises: a frequency identification means for identifying acenter frequency of the electric signal; and a circuit constant controlmeans for controlling the circuit constants of the inductance element,the capacitance element, and the resistance element so that a resonancefrequency of the parallel resonance means matches the identified centerfrequency.
 5. The electromagnetic field shielding apparatus defined inclaim 2, wherein the parallel resonance means is a plate-shaped electricconductor.
 6. The electromagnetic field shielding apparatus defined inclaim 5, wherein the electric conductor includes facing parts forming acapacitor and a loop part forming an inductor.
 7. An electricalapparatus including a shielding apparatus for shielding anelectromagnetic field leaking from a chassis, the electrical apparatusbeing characterized by the shielding apparatus comprising: anelectromagnetic field detection means for detecting an electromagneticfield and converting the electromagnetic field into an electric signal;and a parallel resonance means for causing a parallel resonance whichconsumes power of the electric signal, the electromagnetic fielddetection means being a coil, and the parallel resonance means includingan inductance element, a capacitance element, and a resistance element.8. An electrical apparatus including a shielding apparatus for shieldingan electromagnetic field leaking from an chassis, the electricalapparatus being characterized by the shielding apparatus comprising: anelectromagnetic field detection means for detecting an electromagneticfield and converting the electromagnetic field into an electric signal;and a parallel resonance means for causing a parallel resonance whichconsumes power of the electric signal, the electromagnetic fielddetection means being a coil, and the parallel resonance means being aplate-shaped electric conductor which includes facing parts forming acapacitor and a loop part forming an inductor.
 9. An electromagneticfield shielding apparatus, comprising: an electromagnetic fielddetection means for detecting an electromagnetic field, wherein theelectromagnetic field detection means is a hollow coil for convertingthe detected field into an electrical signal; an electromagnetic fieldgeneration means for generating a counteractive electromagnetic field,wherein the electromagnetic generation means is a coil having either ahollow part identical to that of the electromagnetic field detectionmeans or a hollow part having a center axis identical to that of theelectromagnetic field detection means; and a control means forcontrolling the electromagnetic field generation means so that theelectromagnetic field generations means generates a counterelectromagnetic field which cancels out the detected electromagneticfield, the control means comprising a band-pass filter unit for onlytransmitting a component signal with a specific frequency band out of aplurality of component signals in the electrical signal; and anelectrical supply unit for supplying electricity to the electromagneticfield means according to the transmitted component signal to generatethe counteractive electromagnetic field.
 10. The electromagnetic fieldshielding apparatus defined in claim 9, wherein the electrical supplymeans delays the transmitted component signal by a constant amount ofphase and supplies the electricity according to the delayed componentsignal.
 11. An electrical apparatus including a shielding apparatus forshielding an electromagnetic field leaking from an chassis, theelectrical apparatus being characterized by the shielding apparatuscomprising: an electromagnetic field detection means for detecting anelectromagnetic field; an electromagnetic field generation means forgenerating a counteractive electromagnetic field; and a control meansfor controlling the electromagnetic field generation means so that thecounteractive electromagnetic field cancels out the detectedelectromagnetic field, the electromagnetic field detection means being ahollow coil for converting the detected electromagnetic field into anelectric signal, and the electromagnetic field generation means is acoil which has either of a hollow part identical to that of theelectromagnetic field detection means and a hollow part having a centeraxis identical to that of the hollow coil, and the control meanscomprising: a band-pass filter unit for only transmitting a componentsignal with a specific frequency band out of a plurality of componentsignals in the electric signal; and an electrical supply unit forsupplying electricity to the electromagnetic field generation meansaccording to the transmitted component signal to generate thecounteractive electromagnetic field.
 12. A method for shielding anelectromagnetic field emerging from an opening of a electronic devicechassis, comprising the steps of: placing a coil around the opening ofthe chassis to convert the electromagnetic field emerging from theopening of the chassis into an electrical signal; and sending theelectrical signal to a parallel-resonance circuit, theparallel-resonance circuit comprising an inductor, a resistor connectedin series with the inductor, and a capacitor connected in parallel withthe series connection of the inductor and the resistor.